1. Field of the Invention
The invention relates generally to serial Advanced Technology Attachment (SATA) storage systems and more specifically relates to methods and structure for coupling a SATA host/initiator to a SATA target device through a Serial Attached SCSI (SAS) switched fabric domain.
2. Discussion of Related Art
Storage systems often use high speed serial communications to interconnect host systems and storage devices to achieve longer interconnect cabling and better reliability (better electrical noise immunity). SATA is a common communication protocol and medium for such interconnects—especially in lower cost, smaller storage systems with a limited number of hosts (SATA initiators) and storage devices (SATA target devices). In general, a SATA host is directly coupled with a single SATA target devices (e.g., a single SATA disk drive or a single SATA storage system). SATA port multipliers permit some extension of this architecture to allow a single SATA host to couple with a small number of SATA target devices (i.e., up to 16 devices). SATA port multiplexers further extend the architecture to enable multiple SATA hosts to couple with a single SATA target device. The SATA protocols do not inherently provide for such expansion of a SATA storage architecture and the port multipliers/multiplexors provide only limited extensions to the basic SATA architecture.
SAS communication media and protocols inherently allow much more flexibility in configuring a topology of hosts (SAS initiators) and storage devices (SAS target devices). SAS architectures provide for SAS expanders that enable dynamic switching (a switched fabric) to couple any of several SAS initiators with any of several SAS target devices. Thus, SAS provides improved flexibility and scalability for larger storage systems.
SAS also provides a specific protocol (SATA Tunneling Protocol—STP) to allow connection of SATA target devices to a SAS switched fabric. A SAS initiator supporting STP (also referred to herein as an STP initiator) may then connect with SATA target devices through the SAS fabric (i.e., through one or more SAS expanders). STP, in essence, allows the STP initiator to connect to an identified SATA target through the SAS fabric. An expander directly coupled with the STP initiator follows the SAS protocols using STP to establish the connection (i.e., the route) to an identified SATA target device. Having established the route through the SAS fabric, SATA information is then exchanged between the STP initiator and the addressed SATA target device using standard SATA protocol. This allows for use of less costly SATA storage devices in a SAS architecture storage system. However, STP initiators (e.g., storage controllers) can be more complex and costly than SATA initiators. But present SAS architectures do not allow for SATA initiators (SATA hosts) to couple directly with a SAS fabric-only STP initiators may be coupled with a SAS fabric to exchange information with SATA target devices.
Thus it is an ongoing challenge to provide for low cost, high performance storage systems that include the flexibility of configuration and the scalability of a fully SAS architecture.